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I am a frequent supercharger user and I noticed that my typical Kw is around 72. Yesterday 10-07-2017, I came to charge after going 200 miles round trip with about 30 miles til empty. The speed was around 98 kw until I got to 60% battery.
Today I am plugged in and it is charging at 33kw. Edit: after 10 minutes it jumped up to 72kw. Again same 200 mile trip.

Do we have any answers as to why it is faster some days and others not?

This is the Rancho Cucamonga, CA super charger at Victoria Gardens. Both times the stalls were full. 12/12 taken.

Check the Model S FAQ's thread - you will learn a lot.
The batteries charge at a high rate when almost depleted, but as they get close to 'full' the charge rate slows appropriately. The first owners took extensive records of Supercharging and there is a graph that shows how much you can expect at any point in the charge cycle.
The older Teslas can only charge at a max of 90kW. The newer ones can go up to 135kW.

But the chargers are paired, so...
If you did see 72, you were probably sharing a 'charger pair' where someone had gotten there before you.
When you saw the 33kW jump to 72kW most likely the other Tesla on the pair disconnected leaving you as the 'Prime' charger. Then you would command the max and next person plugging in would get leftover kW until you disconnect.